Color display apparatus having electroluminescence elements

ABSTRACT

A color display apparatus comprises a substrate; thin film transistors formed on the substrate, each of the thin film transistors having a source electrode and a drain electrode; electroluminescence elements respectively formed over the thin film transistors and driven by the thin film transistors, each of the electroluminescence elements having a cathode connected to a source electrode or drain electrode of a thin film transistor, a luminous element layer, and an anode electrode sequentially disposed thereover. A color filter or fluorescent color conversion layer acting as a color element is formed on the side of the anode electrode of an electroluminescence element. The same luminous layer material is used for each display pixel to display a color image.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a color display apparatus thatincludes electroluminescence (hereinafter referred to as EL) elementsand thin film transistors (hereinafter referred to as TFTs) fabricatedon a substrate.

[0003] 2. Description of the Related Art

[0004] Recently, color display apparatuses incorporating EL elementshave been noted as color display devices, in place of the CRTs (CathodeRay Tubes) or LCD (Liquid Crystal Display) panels.

[0005]FIG. 1 is a cross sectional view illustrating a conventional colordisplay apparatus including EL elements and TFT elements. Referring toFIG. 1, thin film transistors are formed on an insulation substrate 2 ofglass or synthetic resin. Each TFT is prepared by successively formingon the insulation substrate 2 a gate electrode 3, a gate insulation film4, an active layer 5 having a source region 6 and a drain region 7, aninterlayer insulation film 8, a source electrode 9 connected to thesource region 6, a drain electrode 10 connected to the drain region 7,and a leveled insulation film 11. Each source electrode 9 is connectedto the anode electrode 37 of an organic EL element. The TFT element actsas a switching element for an organic EL element.

[0006] The organic EL elements are respectively formed on the TFTelements. Each EL element is constructed by successively forming ananode electrode 37, a second hole transfer layer 36, a first holetransfer layer 35, a luminous layer 34, an electron transfer layer 33,and a cathode electrode 32. The anode electrode 37 is made of atransparent electrode of ITO (Indium Tin Oxide) connected to the sourceelectrode of a TFT element. The second hole transfer layer 36 is made ofMTDATA(4,4′-bis (3 methylphenylphenylamino) biphenyl). The first holetransfer layer 35 is made of TPD(4,4′,4″-tris(3-methylphenylphenylamino) triphenylamine). The electrontransfer layer 33 is made of Bebq2. The cathode electrode 32 is made ofMgIn (Magnesium Indium) alloy. The layers except the electrodes are madeof an organic chemical compound. An EL element is formed of therespective organic layers as well as the anode electrode 37 and thecathode electrode 32.

[0007] In the EL element, holes injected from the anode electrode 37 arerecombined with electrons injected from the cathode electrode 32 in theluminous layer 34. Thus, organic molecules of the luminous layer 34 areexcited so that excitons are generated. In the process during whichexcitons disappear, the luminous layer 34 emits light. The emitted lightare radiated out from the transparent anode electrode via thetransparent insulation substrate 2 (in the arrow direction in FIG. 1).

[0008] In the conventional structure, in order to form images, forexample, in three primary colors including R (red), G (green) and B(blue) emitted from organic EL elements, a red luminous material, agreen luminous material and a blue luminous material must be selectedfor the luminous layers 34. For example, porphyrin-zinc complex (ZnPr)is used for red; 10 benzo [h] quinolinol-beryllium complex (Bebq2) isused for green; and azo-methine-zinc complex (AZM) is used for blue.Since the color luminous layers are separately formed by successivelyperforming different steps, the number of fabrication steps isincreased.

[0009] Moreover, since colored light emitted from each organic ELelement is blocked by the TFT area formed on the substrate 2, it isdifficult to obtain sufficiently bright, clear images.

SUMMARY OF THE INVENTION

[0010] The present invention is made to solve the above-mentionedproblems involved in the conventional color display apparatus. It is anobject of the invention to provide a color display apparatus that canrealize an increased color area and clear sharp color images and cansimplify the fabrication process by forming the luminous layer of anorganic EL element with one type of material (or one color material).

[0011] According to the present invention, the color display apparatuscomprises a substrate; thin film transistors formed on the substrate,each of the thin film transistors having a source electrode and a drainelectrode; electroluminescence elements respectively formed over thethin film transistors, each of the electroluminescence elements having acathode connected to a source electrode or drain electrode of a thinfilm transistor, a luminous layer, and an anode layer sequentiallyformed thereover; and color elements respectively arranged on the sidesof anode electrodes of the electroluminescence elements.

[0012] In the color display apparatus according to the presentinvention, each of the color elements comprises a color filter layerformed on a transparent substrate.

[0013] Moreover, each of the color elements comprises a color filterlayer through which light of a predetermined wavelength among emittedlight from the luminous layer can pass.

[0014] Each of the color elements comprises a color filter layer formedon a transparent substrate, the color filter passing light of apredetermined wavelength among beams of light from the luminous layer.

[0015] In the color display apparatus defined in the present invention,the luminous layer of each of the electroluminescence elements is formedof a white luminous material; and the color filter layer includes alayer for red light through which red light within white light emittedfrom the luminous layer passes, a layer for green light through whichgreen light within the white light passes and a layer for blue lightthrough which blue light within the white light passes.

[0016] Moreover, in the color display apparatus according to the presentinvention, each of the color elements comprises a fluorescent lightconversion layer formed on a transparent substrate.

[0017] Each of the color elements comprises a fluorescent lightconversion layer for converting light from the luminous layer into lightof a predetermined wavelength.

[0018] Each of the color elements may be formed of a fluorescent lightconversion layer formed on a transparent substrate, for converting lightfrom the luminous layer into light a predetermined wavelength.

[0019] The color conversion layer of fluorescent materials may be formedof a layer for red light which converts light emitted from said luminouslayer into red light, a layer for green light which converts light intogreen light, and a layer for blue light which converts light into bluelight.

[0020] In the color display apparatus according to the presentinvention, a color filter layer or color conversion layer acting as acolor element is formed on a transparent substrate and can be bonded toan organic EL element. Moreover, since the color element requires onlyone kind of color emitted from the organic EL element, it is notnecessary to use plural kinds of luminous materials to the luminouselement layers of the organic EL element, so that the fabricationprocess can be simplified.

[0021] In another aspect of the present invention, each of said anodeelectrodes in said EL elements is shaped in a comb, mesh, or grid planerpattern.

[0022] Since light in each display pixel is emitted from the colorelements arranged over the anode electrode of the EL element, theluminous or display area become larger than a conventional EL element inwhich the luminescent light is emitted from the substrate on which theTFTs are formed, so that brighter, clearer color image can be displayed.

[0023] In the color display of the present invention, said EL element isable to comprise an organic EL element using an organic material for theluminous layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] This and other objects, features and advantages of the presentinvention will become more apparent upon a reading of the followingdetailed description and drawings, in which:

[0025]FIG. 1 is a cross sectional view schematically illustrating aconventional color display apparatus;

[0026]FIG. 2 is a cross sectional view schematically illustrating acolor display apparatus according to a first embodiment of the presentinvention;

[0027]FIG. 3 is a cross sectional view schematically illustrating acolor display apparatus according to a second embodiment of the presentinvention;

[0028]FIG. 4A is a perspective view schematically illustrating an anodestructure in a color display apparatus according to an embodiment of thepresent invention; and

[0029]FIG. 4B is a perspective view schematically illustrating an anodestructure in a color display apparatus according to an embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] [First Embodiment]

[0031] Next, an embodiment of a color display apparatus of the presentinvention will be described below with reference to the attacheddrawings.

[0032]FIG. 2 is a cross sectional view illustrating a color displayapparatus according to the present embodiment.

[0033] Referring to FIG. 2, the color display apparatus differs from theconventional display apparatus shown in FIG. 1 in the structure. Thatis, the structure where the organic EL elements are reversely placedover the TFT elements. Color filters acting as color elements are placedon the side of the anode electrodes of organic EL elements. Therespective luminous layers are formed of one kind of luminous material(i.e. a luminous material for one color).

[0034] Display pixels 1 are formed by forming thin film transistors andorganic EL elements in a stacked-up structure over an substrate 2, forexample, an insulating substrate such as glass or synthetic resin, or aconductive substrate or semiconductor substrate having an insulationsurface because of insulation thin films such as SiN or SiO₂ depositedon the substrate. The display pixels 1 are arranged in a matrix form toconstruct a color display apparatus. The substrate 2 may be atransparent or opaque substrate.

[0035] The TFT structure is a bottom-gate-type TFT structure where gateelectrodes 3 are placed under the gate insulating film 4. This TFTstructure is similar to the conventional TFT structure usingpolycrystalline silicon films acting as active layers. Hence, therepeated description will be omitted here. The source electrode 9 of aTFT transistor is connected to the cathode electrode 12 of an organic ELelement. The TFT structure may be a top-gate-type TFT structure wheregate electrodes are formed on the gate insulating film.

[0036] Each organic EL element is constructed by successively forming acathode electrode 12 comprised a magnesium indium (MgIn) alloy oraluminum lithium (AlLi) alloy and connected to the source electrode 9 ofa TFT transistor, an electron transfer layer 13 comprised Bebq2, aluminous layer 14, a first hole transfer layer 15 comprised TPD:triphenylamine dimer (4,4′,4″-tris (3-methylphenylphenylamino)triphenylamine), and a second hole transfer layer 16 comprised MTDATA(4,4′-bis(3-methylphenylphenylamino)biphenyl), and an anode electrode 17comprised a transparent electrode such as ITO (Indium Tin Oxide).

[0037] The light emitted from the organic EL element is sent outside thetransparent anode 17 (in the upward orientation in FIG. 2). The anodeelectrode 17 is a common electrode. In each display pixel section, theluminous layer 14, the electron transfer layer 13, and the hole transferlayers 15 and 16 are isolated from adjacent pixel sections by means ofthe insulating film 18. Color filters acting as color elements areplaced on the organic EL display apparatus.

[0038] As shown in FIG. 2, a color filter 22 having R, G and B filtersconfronts the anode electrode 17 and is placed on the transparentinsulating substrate 21 such as transparent film or glass substrate.

[0039] The color filter 22 is securely bonded to the TFT structure witha bonding agent applied on the fringe of the anode electrode 17 of theorganic EL element. The color filter 22 contains color filteringsections each corresponding to a display pixel 1 formed of the ELelements and the TFT elements. A black matrix (BM) 23 may be formedbetween the color filtering sections to block light.

[0040] The organic EL elements respectively emit red, green and bluerays through the color filter 22 in the arrow direction shown in FIG. 2.

[0041] Here, let us now explain the luminous material for the luminouslayer in the organic EL element.

[0042] The luminous material for the luminous layer 14 is selectedaccording to the color element placed over the organic EL element. Inthis embodiment, white light emitting luminous material is used forluminous layer 14 since the color filter (R, G, B) is used as the colorelement.

[0043] A ZnBTZ (zinc benzothiazole) complex is preferably selected as awhite light emitting material for the luminous layer 14. Moreover, alaminated layer having TPD (aromatic diamine)/p-EtTAZ(1,2,4-triazolederivative)/Alq (aluminum quinolinol complex) (where “Alq” actually ispartially doped with neal red being a red luminous pigment) may be usedas a white light emitting material.

[0044] In the present embodiment, the luminous material is formed byusing only one kind of white luminous material. The color displayapparatus is formed by placing a three color filter including R, G and Bover the transparent substrate 21 and bonding the color filter formingsurface with the anode electrode side of the organic EL element. Hence,the fabrication process can be simplified, compared with theconventional manner where three kinds of luminous materials are formedinside the organic EL element to emit three primary colors.

[0045] Moreover, unlike the conventional structure, since light isemitted out as a color beam for a display pixel from the color filtermounted on the anode electrode side, the luminous area per pixel becomeslarge, so that brighter, clearer color images can be displayed.

[0046] [Second Embodiment]

[0047]FIG. 3 is a cross sectional view illustrating a display apparatususing a fluorescent conversion layer acting as a color element.

[0048] The embodiment shown in FIG. 3 differs from the first embodimentin that a color conversion layer 24 of fluorescent materials is arrangedover the anode electrodes 17 and that a blue luminous material, forexample, is used as the material for the luminous layer 14.

[0049] A color conversion layer 24 is formed on the transparentsubstrate 21 such as a glass substrate by evaporating an organicmaterial. The transparent substrate 21 is bonded to the anode electrode17.

[0050] The case will be described below where a blue luminous materialfor a luminous layer in the organic EL element is used.

[0051] The color conversion layer 24 of a fluorescent material has thefunction of converting an irradiated color light into different colorlight. In order to provide a color display by making three primarycolors R, G and B with a blue luminous material acting as a luminouslayer 14, the color conversion layer 24 comprises a material whichconverts blue light into red or green light.

[0052] When blue light emitted from the luminous layer 14 in the organicEL element is converted into red light,4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) isused as the color conversion layer 24R. Thus, the color conversion layer24R converts blue light into red light to obtain a red light emittingdisplay pixel.

[0053] Next, in order to convert blue light emitted from the organic ELelement into green light,2,3,5,6-1H,4H-tetorahydro-8-trifluoromethylquinolidino (9,9a,1-gh)coumarin, for example, is used as the color conversion layer 24G. Thus,the color conversion layer 24G converts blue light into green light toobtain a green light emitting display pixel. Since the luminous layer 14emits a blue light in this embodiment, no light conversion layer 24B forblue is theoretically needed in the display pixel from which a bluelight should be emitted. In the present embodiment, the color conversionlayer 24B is formed to improve the color purity of blue light. The samematerial as that for the luminous layer 14 is used as the colorconversion layer 24B.

[0054] Oxadiazole (OXD), azomethine-zinc complex (AZM), and Al-quinolinemixed ligand complex and perylene may be used as a blue luminousmaterial and a color conversion layer for blue.

[0055] In the embodiment, only one kind of blue luminous material may beused as the luminous layer for an organic EL element. Moreover, a singlelayer in which three kinds of fluorescent conversion materials arearranged side by side is formed over the transparent substrate 21.Hence, this structure allows the fabrication process to be significantlysimplified, compared with the conventional structure where three kindsof luminous materials are formed as a luminous layer in an organic ELelement to emit three primary color rays.

[0056] The case where the luminous layer 14 emits blue light has beendescribed in the above-mentioned embodiment. However, it should be notedthat the present invention is not limited to the above mentionedembodiments. The luminous layer 14 may emit red or green light. A colorconversion layer which converts red light into blue or green light isused for the red luminous layer. A color conversion layer which convertsgreen light into a red or blue light is used for the green luminouslayer.

[0057] Since the color conversion layer formed over the anode electrodecan emit light, the embodiment can provide a larger color light emittingarea, compared from the conventional structure and can display brighter,clearer color images.

[0058] In the above-mentioned embodiments, ITO is used for the anodeelectrode 17. However, the anode electrode may be formed using a vacuumevaporating process or ion evaporating process. For example, aluminumanode electrodes may be formed using an ion cluster method. In thatcase, as shown in FIGS. 4A and 4B, in order to prevent the anodeelectrode 17 from blocking the light emitted from the organic ELelement, the anode electrode 17 is shaped in the comb pattern shown inFIG. 4A or in the mesh or grid pattern shown in FIG. 4B to pass as muchlight as possible. The tooth space of the comb or the mesh or grid gapspace of the mesh or grid are set according to a desired brightness ofthe color display.

What is claimed is:
 1. A color display apparatus comprising: asubstrate; thin film transistors formed on said substrate, each of saidthin film transistors having a source electrode and a drain electrode;electroluminescence elements respectively formed over said thin filmtransistors, each of said electroluminescence elements having a cathodeconnected to a source electrode or drain electrode of a thin filmtransistor, a luminous layer, and an anode layer sequentially formedthereover; and color elements respectively arranged on the sides ofanode electrodes of said electroluminescence elements.
 2. The colordisplay apparatus defined in claim 1 , wherein each of said colorelements comprises a color filter layer formed on a transparentsubstrate.
 3. The color display apparatus defined in claim 1 , whereineach of said color elements comprises a color filter layer through whichlight of a predetermined wavelength among emitted light from saidluminous layer can pass.
 4. The color display apparatus defined in claim3 , wherein each of said color elements comprises a color filter layerformed on a transparent substrate, said color filter passing light of apredetermined wavelength among emitted light from said luminous layer.5. The color display apparatus defined in claim 3 , wherein the luminouslayer of each of said electroluminescence elements is formed of a whiteluminous material; and wherein said color filter layer includes a layerfor red light through which red light within white light emitted fromsaid luminous layer passes, a layer for green light through which greenlight within the white light passes and a layer for blue light throughwhich blue light within the white light passes.
 6. The color displayapparatus defined in claim 1 , wherein each of said color elementscomprises a color conversion layer of fluorescent materials formed on atransparent substrate.
 7. The color display apparatus defined in claim 1, wherein each of said color elements comprises a color conversion layerof fluorescent materials for converting light from said luminous layerinto light of a predetermined wavelength.
 8. The color display apparatusdefined in claim 7 , wherein said each of color elements comprises thecolor conversion layer of fluorescent materials formed on a transparentsubstrate, for converting light from said luminous layer into light of apredetermined wavelength.
 9. The color display apparatus defined inclaim 8 , wherein said color conversion layer of fluorescent materialscomprises a layer for red light which converts light emitted from saidluminous layer into red light, a layer for green light which convertslight into green light, and a layer for blue light which converts lightinto blue light.
 10. The color display apparatus defined in claim 1 ,wherein each of said anode electrodes in said electroluminescenceelements is shaped in a comb, mesh, or grid planer pattern.
 11. Thecolor display apparatus defined in claim 1 , wherein each of saidelectroluminescence elements comprises an organic electroluminescenceelement using an organic material for said luminous layer.